Since a liquid crystal display has such various advantages as operation with low voltage and low power consumption, and capability of miniaturization and thickness reduction, it is widely used for such applications as a monitor of a personal computer or a portable device and a television. As for such liquid crystal display, various modes have been proposed according to an alignment state of liquid crystal molecules in a liquid crystal cell and, conventionally, TN mode, in which such alignment state is formed that molecules are twisted at about 90° from the downside substrate to the upside substrate, has been the mainstream.
In general, a liquid crystal display is constituted of a liquid crystal cell, an optical compensation sheet and a polarizer. The optical compensation sheet is used for the purpose of eliminating image coloring and widening a view angle. As the optical compensation sheet, a stretched birefringent film or a film manufactured by coating liquid crystal on a transparent film is used. For example, JP-A-5-157911 discloses such technique that an optical compensation sheet, which is manufactured by coating discotic liquid crystal on a triacetylcellulose film and aligning and fixing the same, is applied to a liquid crystal cell of TN mode to widen the view angle.
However, a liquid crystal display for an application of television which has a large screen and is assumed to be seen from various angles is strictly required of view angle dependence, and even the manner described above can not have satisfied the requirement. Therefore, liquid crystal displays different from TN mode such as IPS (In-Plane Switching) mode, OCB (Optically Compensatory Bend) mode, and VA (Vertically Aligned) mode are studied.
Incidentally, a cellulose acylate film has such characteristic that it has a high optical isotropy (a low retardation) compared with other polymer films. Accordingly, for some applications where optical isotropy is required, for example, as a polarizing plate, a cellulose acylate film is used.
On the other hand, as an optical compensation sheet (retardation film) for a liquid crystal display, adversely, optical anisotropy (a high retardation) is required.
Therefore, in the technical field of optical materials, it has been a general axiom that a synthetic polymer film is used when a polymer film is required of optical anisotropy, and a cellulose acylate film is used when a polymer film is required of optical isotropy.
AS for a film required of optical anisotropy, for example, as a compensation film of a liquid crystal display, a film having such properties as 50 nm<Re<400 nm, −200 nm<Rth<50 nm and |Nz|<10. With regard to this, there is such example that a film of |Nz|=0.5 and the like is manufactured using a heat-shrinkable film to be used as an optical compensation film (JP-A-5-157911). However, in the method, the process is complicated and, in addition, increase in line speed is difficult because of practice of sufficient heat shrinkage, and a large amount of thermally shrunk film generates, thereby resulting in low productivity and large environmental load. Further, there is such problem that retardation in plane and thickness directions of the resultant film varies largely depending of locations.
On the other hand, as for a cellulose acylate film, an example is disclosed (Japanese Patent No. 3459779), in which cellulose triacetate having substitution degree of 2.92 is stretched to manufacture a film of Re=−50 nm and Rth=−35 nm. However, since the film has a low absolute value of Re, a change in view angle properties can not be suppressed sufficiently.